muscle physiology

skeletal muscle structure

muscle fibers that are long and cylindrical, fused cells with many nuclei, arranged along axis (parallel)

satellite cells- differentiate into muscle for growth or repair

fibers bundles into fascicles surrounded by connective tissue sheath

sarcolemma

cell membrane

sarcoplasm

cytoplasm

sarcoplasmic reticulum (SR)

endoplasmic rediculum

concentrates and sequesters calcium (important for contraction)

unique features of muscle fibers

myofibrils- rod like organelle used for contraction

transverse tubules-allow action potentials to penetrate nearer to internal structure or fibers

lots of mitochondria

glycogen granules

myofibrils

fiber contractile structures

thin filaments =actin

thick filaments= myosin

tropomyosin and troponin (regulatory proteins)

accessory proteins = titan and nebula

Sarcomere

contractile unit of the myofibril

that shortens during muscle contraction, composed of alternating thick and thin filaments

steps leading to skeletal muscle contraction

1. events at neuromuscular junction

2. excitation-contraction coupling

3. contraction-relaxation cycle

molecular contraction

contraction of muscle cells due to actin and myosin sliding past each other

sliding filament theory of contraction- overlapping actin and myosin myofibrils

myosin crossbridges move actin filaments

myosin heads bind to actin and calcium signaling initiates a power stroke where myosin pulls the actin back

power stroke

myosin crossbridge swivels and pulls actin towards M line

end of power stroke

myosin releases actin and resets and binds another actin

heads are not released in unison

initiation excitation-contraction coupling

1. acetylcholine is released from somatic motor neuron

2. ACh initiates an action potential in the muscle fiber

3. action potential in t-tubule alters conformation of DHP receptor

4. DHP receptor opens RyR releasing calcium from SR

5. calcium binds to troponin, allowing myosin-actin binding

6. power stroke of myosin head

7. actin filament slides toward center of sarcomere

8. calcium is pumped back into SR

9. dec in free cytostolic calcium causes calcium to unbind from troponin

10. tropomyosin re-covers brining site, causing myosin heads to release. elastic filaments pull filaments back to relaxed position

skeletal muscle contraction needs steady supply of ATP

glucose is most rapid and efficient store of energy

anaerobic or aerobic

phosphocreatine is backup energy

fatty acids can also be energy source

phosphocreatine

backup energy source of muscles

created from phosphorylation of creatine by creatine kinase

created when muscles are at rest to store for use later

causes of muscle fatigue

central fatigue due to CNS (psychological effects and protective reflexes)

peripheral fatigue due to neuron or muscle worked too hard (most common)

slow twitch fibers

rely on oxidative phosphorylation

resistant to fatigue

have more myoglobin, so darker in appearance

fast twitch fibers

develop tension faster

pump calcium into the SR more rapidly

fast twitch oxidative-glycolytic fibers or fast twitch glycolytic fibers

summation

stronger contraction when the muscle does not relax completely between action potentials

tetanus

summation causes maximal contraction

motor unit

one motor neuron and its muscle

mechanics of body movement

two types: isotonic and isometric contractions

isotonic contractions

move loads

isometric contractions

create force without movement

classification of smooth muscle

by location, contraction patter, and communication with neighboring cells

classify smooth muscle by location

vascular (blood vessels)

gastrointestinal

respiratory (trachea and bronchi)

urinary (bladder, ureter, urethra)

reproductive (ovaries)

ocular (eyes)

classify smooth muscle by contraction pattern

phasic- undergo periodic contraction and relaxation cycles

tonic- continuously contracted

classify smooth muscle by communication with neighboring cells

single unit smooth muscle- more common, forms wall of internal organs, gap junctions

multi unit smooth muscle- cells not electrically linked and must be stimulated independently (iris and ciliary)

12 major differences between smooth and skeletal muscle

1. smooth muscles must operate over a range of lengths

2. within an organ, the layers may tun in several directions

3. smooth muscle contract and relax much more slowly

4. smooth muscle uses less energy to generate and maintain a given amount of force

5. smooth muscle can sustain contractions for extended periods without fatiguing

6. smooth muscles have small, spindle-shaped cells with a single nucleus

7. the contractile fibers are not arranges in sarcomeres

8. contraction in smooth muscle may be initiated by electrical or chemical signals or both

9. smooth muscle is controlled by the autonomic nervous system

10. smooth muscle lacks specialized receptor regions

11. the calcium for contraction comes from the extracellular fluid as well as from the sarcoplasmic reticulum

12. the calcium signal initiates a cascade that ends with phosphorylation of myosin light chains and activation of myosin ATPase

myosin phosphorylation

inc cytosolic calcium released from the SR initiates contraction

calcium binds to calmodulin, initiating a cascade which results in the phosphorylation of myosin light chains (MLC)

myosin light chain phosphatase (MLCP) dephosphorylates myosin

calcium in smooth muscle contraction

sarcoplasmic calcium release activates:

ryanodine receptor (RyR) calcium release channel

IP3 receptor channel

calcium-induced calcium release (CICR)

store-operated calcium channels

cell membrane calcium entry

1. voltage-gated calcium channels

2. ligand-gated calcium channels

3. stretch-activated calcium channels

chemical signaling in smooth muscle

autonomic neurotransmitters and hormones: antagonistic control by sympathetic and para

chemical signals can have different effects (EPI) f

paracrine signals: histamine constricts smooth muscle in airways and nitric oxide relaxes smooth muscles of blood vesselso

compare muscle types

compare muscles